The invention relates to a semiconductor element having a weakly doped continuous interior p-zone which is provided on its surface with an ohmic contact layer and which is adjacent to a weakly doped n-base zone adjacent to which is arranged a highly doped n.sup.+ -zone.
Power rectifier diodes have a p.sup.+ nn.sup.+ -zone structure, i.e. they are composed of two highly doped zones of p or n-type conductivity which lie adjacent to the surface of the semiconductor disc and of a weakly doped zone, generally of n-type conductivity, disposed therebetween. In addition to a sufficiently low forward voltage and a sufficiently low reverse current, fast rectifier diodes must meet the requirement that, during commutation from the forward state, the reverse current peak and its holding time must be small and, in particular, after the reverse current maximum the reverse current must drop gradually (soft recovery behavior). A known means to realize this to a certain extent resides in the reduction of charge carrier lifetime by means of recombination centers, such as gold, platinum or by means of defects produced by electron, gamma or proton irradiation.
One drawback of this method is that the reverse current increases in proportion to the recombination center density and may become too high at the particularly effective recombination levels near the center of the band gap. This effect is very annoying at higher temperatures in connection with gold which, due to its otherwise favorable characteristics, is employed most frequently; it limits the permissible gold concentration and thus the degree of improvement of the recovery behavior.
Moreover, under consideration of the forward voltage, the recombination center density must not be selected too high, i.e. the charge carrier lifetime must not be selected too short. If the ratio of base thickness/.sqroot.D.tau. (where D is the ambipolar diffusion constant, .tau. the charge carrier lifetime at high injection) becomes greater than 2, the forward voltage increases exponentially with N.sub.Rec .about.1/.tau. and becomes too high. With .tau., the stored charge, which is important for the recovery behavior, is also limited downwardly. An additional limitation results from the fact that, in order to avoid too much compensation, the recombination center concentration must clearly lie below the common conductivity doping of the n-base zone which, in turn, is determined by the desired blocking behavior. If this condition is not met, the application of a current pulse will initially produce dynamic voltage peaks which are too high and the forward recovery time--after which the forward voltage approaches the stationary value to within 10% of the initial excess--becomes too long. This also results in a lower limit for the charge carrier lifetime to be set, below which it must not fall. It has been found that this limitation is more drastic in many cases than that set by the stationary forward behavior.
The local distribution of the recombination centers is generally substantially predetermined by the respective technology, i.e. cannot simply be selected on the basis of optimum rectifier behavior. Only with the proton irradiation method can very favorable center distributions be realized. However, proton irradiation is an expensive procedure which has so far not been used in the manufacture of semiconductors. The other irradiation methods also have the drawback that it is generally not possible to us them to set the charge carrier lifetime in the manufacturing plants themselves.
To improve the recovery behavior of rectifier diodes, European Patent No. 0,090,722 discloses a semiconductor structure in which an n-zone of average doping concentration which lies in a range between 10.sup.14 and 10.sup.16 /cm.sup.3 is disposed between the weakly doped n-base zone and the highly doped n.sup.+ -region. A considerable amount of engineering expenditures are required for the production of this intermediate zone since this zone must be produced in the starting silicon by epitaxial growth or must be approximated by additional diffusion steps. In particular, the thus realized improvement of the diode characteristics is only slight because, with the sudden rise in voltage at the time of the reverse current peak, the build-up of the space charge zone is determined primarily by the free holes in the structure flowing toward the anode and these free holes considerably increase the positive space charge. Thus, upon reaching the reverse current peak, the space charge zone does not advance to the zone of average doping concentration in a short time so that this zone is able to influence the recovery behavior only after the reverse current has become small.
Additionally, a pnn.sup.+ -rectifier diode has become known (IEEE Trans. Electron Dev. ED-31, 1984, page 1314) in which the doping concentration .rho. and the thickness w of the p-zone on the anode side is small in laterally small channel regions, e.g. .rho.=5.times.10.sup.15 /cm.sup.3 and w=1 .mu.m, but considerably larger in the other surface regions, e.g. .rho.=4.times.10.sup.18 /cm.sup.3 and w=5 .mu.m. The lateral expanse of the channel regions lies in an order of magnitude of 2 .mu.m. As a result, the p-zone regions of low doping concentration and thickness have a low concentration of injected charge carriers on the p-side of the n-base zone during forward operation and thus the recovery behavior is good. Under a forward blocking load, the channel regions are statically shielded so that the space charge zone does not penetrate to the surface in spite of the low doping concentration and thickness of the p-zone in these regions. One drawback of this structure is that in this way, blocking capabilities up to only about 150 V can be realized and the structure is not suitable for high voltages also because the forward voltage then comes too high. Another drawback is that individual deviations and faults in the lateral p-zone structure may result in a substantial loss of blocking capability so that the number of rejects of large-area diodes of this type becomes too large for economic use.
In addition to a short recovery period, fast thyristors must also meet the requirement that, upon commutation from the forward conducting state, the reverse current peak and the reverse current integral must be small. Similarly to the situation described above for rectifier diodes, this requirement is realized only insufficiently in prior art thyristors by doping them with recombination centers since the use of such thyristors is limited due to the secondary requirement for sufficient forward conduction characteristics and a sufficiently low reverse current.